1. Field of the Invention
The present invention relates to an image position matching method and apparatus in which matching the positions of the same structural components contained in two or more images is improved.
2. Description of the Related Art
It is a common practice in a wide variety of fields to read and compare a plurality of two or more images taken of the same subject in order to discern the difference between images, and carry out an inspection of the subject based on the thus discerned difference, etc.
In the industrial products manufacturing industry, for example, images taken of a product when it is new and images taken of the same product after it has been subjected to an endurance test are compared, and attention is focused on the area showing the biggest difference so that an examination as to whether or not there are areas requiring improvement in product endurance can be carried out. In the medical field, doctors read and compare a plurality of radiation images of the diseased part of a patient taken in a time series manner in order to ascertain the course of the disease and determine the proper treatment.
In this fashion, reading and comparison of a plurality of images is carried out daily in a wide variety of fields. To facilitate such comparative reading, there are cases in which the two or more images are displayed on a display means such as a monitor. In other words, the image is converted to luminescence and density signals and displayed on a monitor, etc.
Although the area of the most interest to the viewer of such images is the area wherein the greatest difference is shown, when the two or more images that are to become objects of comparative reading are compared, it is a common practice to simply line them up and display them. However, under such conditions, the smaller the difference between the images is, the more difficult it becomes to discern, leading to a desire for an improvement in comparative reading performance.
The positions of the same structural components appearing in each of the two images are coordinated by transforming at least one of the images, after which the pixels of both images are coordinated (positions coordinated), and by performing a subtraction process, a difference image representing the difference between the two images is obtained. By displaying the subtraction image on a CRT, etc. display means, the reader of the image can easily discern the difference between the two images.
However, in an image such as a radiation image of human chest, in which contains tissues of different densities, such as ribs, backbone, muscle tissue, organs and other soft tissues, are contained in the same image, there are cases in which different values for the direction of the positions of the bone and soft-tissue structures are shown, corresponding to the amount of change in the position (the standing position when the image was obtained, the direction faced, etc.) of the subject (human body). Conventionally, position matching has been performed based on the amount of change and the direction of change, without regard as to whether the change was due to a change in the bone or the soft-tissue structures. Accordingly, because the differences based on the bone and soft-tissue structures become mixed in the amount of change and direction of change used to perform position matching, for cases in which there has been a difference in the amount of change in the bone structures and the soft-tissue structures, accurate matching of positions between portions of the images has not been possible, and there have been cases in which a subtraction image has been formed in which both bone structure artifacts and soft-tissue structure artifacts have remained; it has been difficult to achieve satisfactory diagnosis using subtraction images in which different types of artifacts from various structural components remained. For example, for cases in which bone structures and soft-tissue structures are adjacent, or stacked (for example, a case in which a rib and a blood vessel are stacked), the direction of change of the bone structure, for example, to the right, the direction of change of the soft-tissue structure next to the bone structure is in the opposite direction, to the left, and if these are employed for position matching the positions become even less exact, and in a worst case scenario, even more than one rib becomes mispositioned.